1. Field of the Invention
The invention relates in general to an encapsulation structure of double sided organic light emitting device (OLED) and method of fabricating the same, and more particularly to a compact encapsulation structure of OLED being resistant to moisture and present a compact appearance.
2. Description of the Related Art
Use of organic light emitting device (OLED) in the flat panel displays possesses several competitive advantages, such as self illumination, high brightness, wide viewing angle, vivid contrast, quick response, broad range of operating temperature, high luminous efficiency and uncomplicated process of fabrication. Thus, the OLED represents a promising technology for display applications and has receives the worldwide attention in the recent years.
The typical structure of OLED is mainly constructed by interposing an organic light emitting layer between an anode and a cathode. A hole injection layer (HIL) and a hole transport layer (HTL) are interposed between the anode and the organic light emitting layer. An electron transport layer (ETL) is interposed between the cathode and the organic light emitting layer. This laminated structure of OLED facilitates the electron flow from the cathode to the anode. The organic light emitting layer can be divided into tow groups according to the materials in use. One group is a molecule-based light emitting diode, substantially comprising the dyestuffs or pigments. The other group is a polymer-based light emitting diode, also known as “LEP” (i.e. light emitting polymer). Whether the organic light emitting layer is molecule-based or polymer-based, the moisture shock has the considerable effect on the image quality displayed by the device. Those common effects includes the short operation life, low emission efficiency of the organic light emitting layer caused by the material degradation, and the “dark spot” (i.e. the spot of the light emitting portion unable to emit the light which results from a lack of current flow at the spot) caused by the adhesion failure between the organic light emitting layer and the cathode. Therefore, it is necessary to protect the OLED from the moisture intrusion.
Additionally, it is a trend to make a displaying apparatus, for example, cellular phone, personal digital assistance (PDA), digital camera and laptop, with doubled sided OLED for showing more information on the displaying apparatus simultaneously. The doubled sided OLEDs can be divided into three types according to the structural differences. The first type uses one OLED with two transparent electrodes to display two opposite images. In the second type, a bottom-emission OLED and a top-emission OLED are formed at the same substrate for displaying two independent images, which is low production cost but small aperture ratio of pixels of the displaying apparatus. Also, the technique of the second type OLED is immature and not suitable for fabrication on the mass production scale so far. In the third type, two bottom-emission OLEDs are formed at the different substrates. After the substrates are well assembled, the two different images can be presented independently. It is easier to prepare the third type OLED on the mass production scale. However, for assembling and encapsulating the third type OLED, the techniques of solid thin film packaging and/or thin-film desiccant disposing may be required, which both are still developing and not ready for application on the mass production scale.
Generally, the thickness of the desiccant is about 100 μm at least. If the third type doubled sided OLED is assembled and encapsulated according to the design for assembling and encapsulating the conventional OLED (as shown in FIG. 1 and described later), the thicker sealing glue will be needed so that moisture is easy to enter and damage OLED (as showing in FIG. 2). In the following paragraphs, the methods for assembling and encapsulating the single sided OLED (i.e. the conventional type) and the double sided OLED (i.e. the third type) are further described.
FIG. 1 (related art) illustrates a conventional encapsulation structure of single sided organic light emitting device (OLED). As shown in FIG. 1, an organic light emitting device (OLED) 3, formed on a glass substrate 2, comprises an anode (made of indium tin oxide (ITO), indium zinc oxide (IZO) or cadmium tin oxide (CTO)), an organic light emitting layer and a cathode. Also, a cover such as a glass sealing case 4 is assembled with the substrate 2 by a sealing glue 6, thereby providing an internal space 5 between the glass sealing case 4 and the OLED 3. The sealing glue 6 is typically in a thickness of 10-20 μm. The dried inert gas is enclosed in the internal space 10. Also, a desiccant 7 is attached to the glass sealing case 4 for absorbing the moisture.
FIG. 2 (related art) illustrates a conventional encapsulation structure of double sided organic light emitting device (OLED). The encapsulation design of FIG. 2 is the third type of the structures mentioned before. For displaying two individual images, the first and second OLEDs 23, 24 are formed on the first and second substrates 21, 22, respectively. The desiccant (mainly composed of calcium oxide (CaO)) 27 is attached to either the first OLED 23 (as shown in FIG. 2) or the second OLED 24. Afterward, the first and second substrates 21, 22 are assembled by the sealing glue 26 to complete the encapsulation. However, this conventional encapsulation design may cause damage to the first OLED 23/the second OLED 24 during the assembly. Also, the thicker sealing glue 26 contacts the atmosphere with larger area, so as to increase the opportunity for moisture to get inside the encapsulation structure.